Dielectric elastomer devices (DED), whether actuators (dielectric elastomer actuators, or DEA) or generators (dielectric elastomer generators, or DEG), are useful transducer devices which may be used for converting electrical energy to mechanical energy or vice versa. DEDs have some unique properties with respect to traditional actuators/generators, such as being inherently compliant, which make them particularly suitable for some applications.
A DED typically comprises a dielectric elastomer membrane 11 sandwiched between opposing compliant electrodes 12. The dielectric elastomer membrane 11 is compressed by electrostatic pressure when a high voltage is applied across the electrodes 12 in the manner of a capacitor, causing planar expansion of the membrane from an uncompressed or contracted state to a compressed or expanded state.
As is the case with traditional transducers of the prior art, in many applications it is useful or necessary to have some feedback regarding the state of the device. This feedback may be used for closed-loop control, or detecting whether a device is operating near its limits (i.e. to identify to limit the risk of imminent failure of the device), for example. Traditional transducers of the prior art would thus typically be fitted with some external sensors (such as micro-switches, strain gauges, optical sensors etc.) to directly measure or detect movement of the device. The required external sensors increase the component count and complexity of the actuator.
One advantage of DEDs is that feedback on the state of a DED can be obtained solely from measurable electrical characteristics of the actuator/generator transducer itself, referred to as self-sensing, preferably dynamically (e.g. as a DEA is actuated or a DEG is mechanically deformed). In particular, because the membrane of a DED is preferably volumetrically incompressible, it is possible to relate a change in capacitance between electrodes of the DED to changes in the physical geometry of the DED. External sensors are therefore not required. A self-sensing DED is thus multifunctional—by monitoring its geometry, the DED can act as a strain or pressure sensor while simultaneously functioning as an actuator or generator (i.e. a self-powered sensor). An estimate of the capacitance of the DED can also be used to provide additional useful feedback data regarding the electrical state of the DED including the charge and leakage current which may be indicative of the health of the DED. External sensors do not directly provide any feedback regarding the electrical state of a DED.
As the charge on a DED increases, so too does the electric field and electrostatic pressure. If the electric field is allowed to grow too large the DED will undergo dielectric breakdown. When this happens the charge on the DED will be rapidly discharged through the thickness of the membrane 11, generating significant heat and often resulting in catastrophic failure of the DED. Monitoring the leakage current can also enable detection of the precursors to dielectric breakdown and failure of the DED.
Due to the high voltages necessary for efficient DEDs, implementing capacitive self-sensing is not as simple as applying the capacitive sensing techniques commonly applied in other fields. Nonetheless, self-sensing methods have previously been developed and used to provide feedback in DEA systems.
International Patent Publication No. WO 2009/01515, for example, discloses a self-sensing dielectric actuator system in which a relatively high frequency sensing signal is superimposed on a low-frequency actuating signal by a signal mixer, and deformation of the actuator is estimated from changes in the sensing signal.
International Patent Publication No.'s WO 2010/095960 and WO 2012/053906 each disclose a method and system for estimating the capacitance between opposing electrodes of a DEA dynamically (i.e. as the DED is actuated) by measuring only the voltage difference between the electrodes and a series current supplied to the DEA. Using the capacitance estimate, the charge and leakage current can also be estimated.
However, there are several potential disadvantages to such systems and methods.
Firstly, due to the high voltages typical in DED systems, the prior art methods require relatively expensive high voltage electronic components in introducing the required oscillation in the actuation signal, whether it be a floating oscillating signal source or a signal mixer. The oscillation may alternatively be generated by the actuation signal source itself (e.g. using pulse width modulation), but this complicates the design.
Secondly, the methods cannot be easily applied to implement self-sensing in a DEG system. The method and system of WO 2010/095960, for example, requires the voltage of the DEA to be oscillated at least during self-sensing. This oscillation can be provided by an oscillating high voltage supply or using high voltage switching to rapidly charge/discharge the DE actuator. The self sensing method thus induces a small scale voltage oscillation across the DEA while simultaneously actuating it. The response of the DEA to the induced ripple enables its strain state and electrical state to be derived.
However, a DEG generator operates somewhat differently to a DEA, and the control circuits/strategies for controlling the DEG are not conducive to the DEA self-sensing systems and methods of the prior art.
A DEG generates electrical energy by increasing the electric potential energy stored in it. The steps to achieve this are illustrated diagrammatically in FIG. 1. Starting from the top of FIG. 1, mechanical energy 10 is initially applied to the DEG 11 by stretching it. This results in a planar expansion of electrodes 12 and an orthogonal compression of the membrane 13, leading to an increased capacitance. Electrical energy 14 is then input to the DEG by charging or priming from an electric power source (not shown) so that opposing electrodes 12 become oppositely charged. Relaxing the DEG will convert the mechanical energy into electrical energy by forcing apart the opposite charges (+ and −) on opposing electrodes 12, and forcing the like charges on each electrode 12 closer together due to the planar contraction thereof. The electrical energy 14 is extracted and the cycle repeats.
To provide the priming charge, the DEG system preferably comprises a self-priming circuit such as that disclosed by International Patent Publication No. WO 2011/005123. However, the self-priming circuit does not provide a constant connection between a high voltage power source and the DEG as required for the self-sensing DEA method of WO 2010/095960.